When I first started working at a job, I used to work as an IT administrator. At one point, I even installed cat-5 cable through an entire 5000 sq. feet office building. That was a lot of fun as I had to access the office ceiling for a whole day.
Anyways, a Cat-5 ethernet cable is nothing more than “enhanced” phone cables. The phones usually use Cat-3 cables, which are slightly thinner wires and is more prone to electromagnetic interference. Of course, Cat-5 cables can actually be better for your phone lines because they are much more durable and designed for protection against eletromagnetic interference.
A cat-5 ethernet cable has 8 wires. One LAN line requires only two wires so as long as you don’t use all of the wires, you will still have plenty for using the rest for phone lines.
I won’t get into the details of using an ethernet cable as phone line but you can read this instructables that shows you how.
The solution is to run both ethernet and phone over the same existing cat-5 cable. Every wall jack becomes two jacks, one RJ-11 for phone and one RJ-45 for ethernet. This neat hack could save you a lot of money, as you only have to buy new wall plates and jacks rather than wall plates, jacks, and hundreds of feet of wire.